This invention relates to cathodes and has particular but not exclusive reference to cathodes for use in copper electrorefining and copper electrowinning.
Copper refining by electrolytic methods has been known for many years in which pure copper is electrodeposited at the cathode of an electrolytic cell in which the anode is a sacrificial impure copper anode and which is consumed during the electrolysis. It has been generally the practice in a first stage to electrodeposit a thin layer of pure copper onto a specially prepared mother plate, in a second stage to strip off the freshly deposited pure copper from the mother plate in the form of a thin sheet or starter sheet, and in a third stage to use this starter sheet as a cathode in another cell in which a further thick layer of pure copper is electrodeposited on the cathode. More recently, titanium has been used as the material for the mother plate in this process. A second process is to build up a thick deposit of pure copper directly onto a titanium cathode from which it is subsequently stripped as a thick plate, thereby eliminating the first and second stages of the previous process.
When titanium is used as a material for either the mother plate in the first process or the cathode in the second process, each mother plate or cathode is connected to the current carrying busbar by means of a hanger bar which stretches across the electrolytic cell and contacts the busbar located on one side (or both sides) of the cell. Previously, these hanger bars have been formed of copper and the connection between the copper and the titanium mother plate or cathode was by means of bolts or rivets. The electrical contact between the mother plate cathode (hereafter referred to simply as the cathode) and the hanger bar was found to be inconsistent. The cathode and hanger bar are tightly held in the vicinity of the bolts or rivets but elsewhere the surfaces became slightly separated. The separation occurred as a result of mechanical deformation, or differential thermal expansion. When a separation between hanger bar and sheet had been formed, splashes of electrolyte were forced into the gaps, and on drying out, left crystals of, for example, copper sulphate in the gap. When the gap is closed by other deformations, the crystals prevent full closure. Further movement allows more material to build up in the gap and as a result, the gap is widened out by a ratcheting type of action. Clearly, the reduction of surface contact area resulted in an increase in resistance of the joint.
The old copper to copper joints were of high quality and the electrolyte splashes had a cleaning action on the copper. However, the electrolyte has no cleaning action on the titanium. Additionally, the surface oxide film formed on the titanium interface interferes with the electrical contact. The surface film tends to grow if the titanium is heated as a result of the resistance across the titanium copper interface. With the currents which have been used to date, the electrical contact problem has been solved by utilising a greater number of bolts or rivets to increase the electrical contact. Over the past five years or so, however, the use of higher currents in electrolytic refining has meant that serious problems of contact resistance have developed.
As many cathodes are used in parallel, and the current supplied is constant, if the resistance of one of them increases, it receives less current. Not only does this result in a lower rate of deposition on that cathode, it also increases the current passing through the remainder of the cathodes. This can cause the next higher resistance cathode to become over-loaded and to overheat, distort and increase in resistance. This results in a further increase in current through the remainder of the cathodes and a cascade of failures can then occur.
The heating of a cathode can, in addition to increasing the load upon the remainder of the cathodes, distort the cathode. Any small amount of distortion is compounded by extra local growth where the cathode approaches the anode. This can then result in nodular growth of deposit on the cathode with a rapid build-up of a deposit on the cathode, and a short between the cathode and anode.
Also, since the current loss in heating the joint between the cathode and the hanger bar is a complete waste of energy and consequently money, this factor has an important bearing on the economics of electrolytic refining. The heat generated also distorts the joint and the sheet.
A very elegant solution to the problems associated with these earlier cathodes has been proposed in which the hanger bar is in the form of a titanium-clad copper bar in which the copper is metallurgically bonded to the titanium. The main sheet of titanium is then spot-welded along one edge to the outer sheath of the hanger bar and the cathode then suffers from none of the problems mentioned above. The solution is clearly very elegant since it solves in one go the great majority of the previous problems. However, the product is relatively expensive to manufacture.
The product may be made by placing a copper billet in a titanium cylinder and placing a further copper sheath on the outside. Copper end lids are then welded to the copper outer sheath and the product is then extruded at a high temperature to metallurgically bond the copper and titanium and the outer copper layer is then pickled off. The round starting billet is extruded straightaway into a substantially rectangular shape. However, this results in an excess of titanium at the ends of the rectangle when seen in cross-section. The titanium is mainly needed at those points where spot-welding occurs and excess titanium at the ends is a waste. To enhance the strength of the hanger bar, it is necessary to cold work the product to harden the copper. It is not possible to cold draw the product since there is a danger of galling of the titanium on the die and special surface treatments would be needed. Since the product cannot be cold drawn, it is difficult to control the final dimensions.
It is necessary to remove the titanium sheath at the ends to enable proper electrical contact to be made with the busbars and if the hanger bar has to fit into a Baltimore groove, the starting size has to be sufficiently large to enable all of the titanium to be removed and still leave an inner core of sufficient size and shape to fit properly into the groove.
The majority of these problems are overcome by the present invention.
By "film-forming metal" as used herein is meant a metal chosen from the group titanium, niobium, aluminium tantalum, hafnium, or alloys of these metals.